Room temperature-vulcanizing polyorganosiloxane compositions which cure at room temperature by moisture in air to give rubber-like elastomers are well known as condensation-type room temperature-vulcanizing liquid silicone rubbers (RTV silicone rubbers), and are extensively used in various industrial fields. Such a composition consists essentially of a polyorganosiloxane base polymer having a hydroxyl group at both ends thereof, a silane crosslinking agent having hydrolyzable groups, and a curing catalyst. So-called one-package RTV silicone rubbers prepared by mixing all those ingredients and packaging the mixture to protect it from moisture are frequently used as a silicone sealant, silicone coating material, and the like in various industrial fields including manufacturing industries and the building industry and also in domestic repair applications, because cured elastomers obtained from the one-package RTV silicone rubbers have excellent properties inherent in the silicone, such as heat resistance, cold resistance, weatherability, and electrical insulation properties, and because the RTV silicone rubbers have bondability to various materials and easily cure into rubber-like elastomers by exposure to the air.
Such one-package RTV silicone rubbers have various features depending on the kind of the hydrolyzable silanes used as a crosslinking agent. Hydrolyzable silanes practically used in the one-package RTV silicone rubbers include acetoxysilanes, alkoxysilanes, aminoxysilanes, amidosilanes, enoxysilanes, and ketoximatosilanes. Of these, acetoxysilanes enable RTV silicone rubbers to have excellent curability and to show excellent bondability particularly to glasses, but there is a problem that since acetic acid is released during the curing of the rubbers, a strong irritative odor is given off and metals, marble, and other substrates are attacked. RTV silicone rubbers using alkoxysilanes have features that they are free from an irritative odor and do not attack metals, and they rarely cause polycarbonate substrates to suffer from solvent cracks. RTV silicone rubbers using aminoxysilanes or amidosilanes have a feature that cured elastomers having low modulus can be obtained, but they are defective in slow curing. RTV silicone rubbers using enoxysilanes have quick-curing properties and do not attack metals, but there is a problem that they suffer yellowing in an uncured state and they also have a cost problem. One-package RTV silicone rubbers using ketoximatosilanes as a crosslinking agent emit no irritative odor, have appropriate curability, and rarely attack metals, except copper-based metals, and such silicone rubbers are advantageously used in general-purpose industrial fields an building fields as an RTV silicone rubber having well-balanced properties and an appropriate cost.
For this reason, various studies have so far been conducted of the one-package RTV silicone rubbers using ketoximatosilanes as a crosslinking agent.
For example, JP-B-46-11272 discloses a method for imparting bondability. (The term "JP-B" as used herein means an "examined Japanese patent publication".) JP-B-62-17624 discloses a composition containing an organoaluminum as a curing catalyst. JP-A-60-161457 discloses a composition using a titanium chelate compound to improve elastic modulus and bondability. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) JP-B-63-19537 discloses a composition using a tetrafunctional oximesilane.
However, the one-package RTV silicone rubbers using ketoximatosilanes as a crosslinking agent have had a serious problem to be solved. The problem concerns the property of following up the movement of joints in the course of curing. Once a silicone sealant, which is an elastic adhesive to bond two materials, is cured, the cured sealant can satisfactorily follow up the movement of the joint within the elastic limit thereof. However, in the course of curing, a cured thin layer formed on the surface of the sealant applied at the initial stage of the curing often suffers cracking or wrinkling, and this has been a serious problem from the standpoint of sealing performance and also has not been preferable from the appearance standpoint.
The movement of joints takes place easily due to the expansion and contraction of the adherends with changing temperature, or due to vibrations, external forces, etc., and particularly in the case of buildings, it is difficult to completely prevent such joint movement. Of various one-package RTV silicone rubbers, those using ketoximatosilanes as a crosslinking agent have a tendency to undergo cracking as compared to the others, because they show a middle degree of curability and because they are in a plastic state in the course of curing.
As a means for overcoming the above problem, the present inventors proposed in JP-A-(Hei)1-121366 a composition which can effectively overcome the above problem by addition of an oragnotitanium compound. Although the cracking in the course of curing is eliminated by the addition of an oragnotitanium compound, such composition is still insufficient in heat resistance and dielectric strength. Hence, there has been a desire for an improvement of the composition for use in more severe environments that also require a higher degree of heat resistance and dielectric strength.